Earth's finishing touch came with a wallop when a Mars-sized hunk of real estate crashed into the fetal planet some 95 million years after the birth of the solar system -- later than some astronomers thought -- sending up debris that eventually formed moon, a new study shows.
The finding, based on 259 computer simulations of asteroid crashes and the resulting buildups of Earth, helps resolve a long-standing debate about when the moon-forming impact occurred.
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"Earth was put together piece by piece. It didn't just appear all at once. And the last piece was probably when something the size of Mars -- about 10 percent of Earth's mass -- hit Earth," astrophysicist John Chambers, with the Carnegie Institution for Science in Washington, D.C., told Discovery News.
"Most of that material stayed on Earth, but some of it got blown into space, along with some stuff from Earth, and that then coalesced to form the moon," Chambers said.
Previous studies used the natural breakdown of telltale radioactive trace materials to estimate how much time passed between the formation of the solar system 4.6 billion years ago and the big splat that launched building blocks for the moon into Earth's orbit.
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But those studies were not conclusive, with some results showing the impact occurring as early as 30 million years after the birth of the solar system.
The new research, which favors a later date, paves the way for scientists to get on with an even meatier question: Why the big discrepancy between Earth's prolonged formation and the speedy evolution of Mars?
Analysis of Martian meteorites shows the planet formed in just a few million years, a finding supported by the new computer simulations.
That debate is likely to continue until samples are brought back from sister planet Venus, which, like Earth, likely had a long incubation.
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"There's a lot of uncertainty in what (the solar system's) proto-planetary disk looked like," lead researcher Seth Jacobson, with the Cote d'Azur Observatory in Nice, France, wrote in an email to Discovery News.
"This disk is filled with large planetary embryos and small planetesimals which collide and eventually build the planets. We don't know what shaped this disk, how fast it grew, how long it lasted .... If we could compare Earth and Venus this would tell us a lot about conditions in the proto-planetary disk," he wrote.
Learning more about how the solar system and its planets formed also provides context for fruitful searches for other potentially habitable systems.
The research is published in this week's Nature.